This invention relates to a system for encrypting and decrypting data signals.
As is appreciated by those of ordinary skill having the benefit of this disclosure, the transmission of encrypted data requires both sender and receiver to have access to information about the encryption method to be incorporated into both the encryption method and the decryption method. An operator receiving a Morse code signal, for example, is able to decrypt the signal because of his a priori knowledge of the meaning of the signal patterns.
Likewise, a signal can be encrypted using a cipher table or encryption table when both sender and receiver have access to a copy of the table. A simple and well-known example is an encryption table such as shown in FIG. 2. In such a table, each letter of the alphabet in the clear-text signal is encrypted by locating it in in the first column of the table and translating it to the corresponding letter in the second column of the table. The letters in the second column comprise the letters of the alphabet distributed at random. In the example shown in FIG. 2, the word "cab" would be translated to "zmi." A greater number of permutations can be obtained for the second column by encrypting each clear-text letter into two-letter sequences, three-letter sequences, and so forth. And of course, nonletter characters such as numbers can also be encrypted along with letters in a similar manner.
It is also well-known to increase the security of an encrypted transmission by utilizing different encryption tables. A familiar example is the one-time pad, in which a series of encryption tables is used, with each encryption table being used once, or on one day, etc., then discarded.
The simple translation method described above is but one method of encryption. Many complex and sophisticated encryption techniques are also known. Many such techniques require considerable work and time, even when performed on a computer, to generate an encrypted signal. Computer encryption using such methods can be very hardware intensive, requiring considerable memory to be used effectively.
In many situations the added security provided by such complex methods is worth the cost in time and resources. In other situations, however, simpler methods that provide some lesser level of security can be more cost-effective.